2020 Issue

32 Another impact with long-term implications is the genera- tion of higher than normal sediment loads in the flows from the watershed. The Dollar Ridge Fire burned the watershed tributary to Starvation Reservoir. The Central Utah Water Conservancy District operates the Duchesne Valley Water Treatment Plant (DVWTP) that draws water from Starvation Reservoir for treatment and distribution. The plant is the only water supply for parts of Duchesne County. After the fire and initial assessment, CUWCD was con - cerned with the operation of the plant in the impacted sys- tem. The pollutants of concern included turbidity, nutrients, algae growth, organics, disinfection byproducts (DBPs) and dissolved oxygen. Turbidity from increased sediment has the potential to impact the treatment and filtration systems, as well as impact fish within the reservoir. Nutri - ents, such as phosphorous, increase the potential for algae growth, which then interferes with the filtration processes and has the potential to cause cyanobacteria blooms. Increased organics in the water cause more disinfectant demand, which leads to increased disinfection by-products (DBPs), and can also change both the taste and odor of the water delivered to end-users. Increases in suspended sol- ids can cause lower dissolved oxygen, reducing the oxygen available for fish within the reservoir. Figure 5 shows the results of sampling on several days at one of the monitor- ing locations in the Starvation Reservoir. Figure 5 — Turbidity Monitoring Locations and Results on Starvation Reservoir Watershed Recovery From Fires The vegetation of chaparral communities has evolved to a point where it requires fire to spawn regeneration. Many studies have shown an increase in runoff and erosion rates the first year following a fire, with recovery to pre-fire rates generally within five years (Wright and Bailey 1982). The timing and extent of recovery are highly dependent on pre- cipitation, slope, and vegetation type (Branson et al. 1981, Wright et al. 1982, Knight et al. 1983, Wilcox et al. 1988). Pierson, Jr. et al. (2003) noted that water repellency of the hydrophobic water layer deteriorates over time, resulting in a gradual recovery in the infiltration capacity of the soil. Other studies have numerically quantified the Ainsworth and Doss (1995) qualitative summary. Pierson, Jr. et al. (2003) studied two watersheds in Idaho that were severely burned. They note that virtually all vegetation and litter was consumed during the fire. Bare ground for all burned sites was greater than 95% resulting in increased soil exposure to the erosive forces of raindrop impact and overland flow. It took two growing seasons and three winters for litter accumulation to reduce the amount of bare ground on the burned sites to near 50%. Watershed vegetation recovers to 90% of the pre-fire condition after five years. These results are consistent with the results of the other research- ers, both quantitatively and qualitatively. Local Impacts and Civil Engineering Fires and post-fire impacts often impact built infrastructure and utility systems that are operated by civil engineers. These impacts include higher runoff volumes, debris flows, and impacts on water quality. One example of impacts to facilities in Utah in 2018 includes impacts on watersheds and water supply systems. Figure 4 shows debris from the fire was washed down to the culvert during a summer thunderstorm. Figure 4 — Impacts to Culvert on Strawberry River The water intake from Starvation Reservoir near the DVWTP intake is usually less than 3.0 NTU. As a direct fil - tration plant, the DVWTP cannot treat high turbidity water under Utah rule R309-530-5.3.g. The rule requires that the plant be designed and operated so that it will automatically shut down when source water turbidity is 20 NTU for more than three hours, or when source water turbidity exceeds 30 NTU at any time. After the fire and resulting debris flows, high turbid - ity water channeled across the bottom of the reservoir